The present invention relates to ground fault circuit interrupters (GFCIs) and, more particularly, to built in test (BIT) circuitry which increases operational reliability of GFCI devices.
Ground fault circuit interrupters (GFCIs) were developed to meet a great need for a device which is capable of detecting the presence of abnormal current flow, e.g., current flow from a phase line to ground, and immediately interrupting power to a faulted line in which the abnormal current is detected to protect persons from electric shock, fire and explosion. To thoroughly protect human life, electric circuit breakers should detect such faulted currents on the order of 3 to 50 mA corresponding to load currents ranging on the order of 10 to 100 A.
Prior to GFCI development, differential circuit breakers were known and used in certain European countries to provide ground fault protection. Differential circuit breakers include a differential transformer with a core through which two conductors of the electrical circuit being monitored pass. The two wires act essentially as primary windings. The differential transformer also includes current interrupting contacts, which, in the event of a line to ground short circuit or an abnormal leakage current to ground, are forced to a high impedance state, i.e., an open state. The state of the contacts is controlled by a semiconductor device which is energized by a secondary of the differential transformer. It was found that such devices were, however, unacceptable, due to their current sensing insensitivity and, therefore, ineffectiveness in ensuring complete protection for human life.
GFCIs evolved from differential circuit breaker technology. GFCIs developed as ground sensors including a circuit breaker connected between a power source and a load; the power source is connected to the load through the GFCI via a neutral and phase conductor. The GFCI also includes a differential transformer connected across the neutral and phase conductors. The circuit breaker is actuated when the differential transformer senses that more current is flowing into the load from the source through the phase conductor than is flowing back to the source through the neutral conductor. Primary and secondary windings are included within the differential transformer which provides a means for sensing the current. A tertiary winding is disposed proximate the neutral conductor in the vicinity of the load whereby a current is induced therein in the event of a grounding. If the induced current is large enough, the circuit breaker contacts are forced open.
Similarly, a ground fault protective system is known which includes a differential transformer comprised of a toroidal core through which each of two line conductors and a neutral conductor pass to form primary windings of at least one turn. A secondary winding of the differential transformer serves as an output winding and is connected to a GPCI circuit. A trip coil of a circuit breaker having a plurality of contacts in line with the conductors of a distribution circuit is energized with a minimum current. A pulse generator is coupled to the neutral conductor for producing a high frequency current therein upon grounding of the neutral conductor between the differential transformer and the load. The high frequency current is produced by the periodic firing of a diac when a voltage on a capacitor connected thereto is applied to the output winding. The pulses induce voltage pulses in the neutral conductor passing through the transformer core. The induced voltage pulses do not effect the current balance in the distribution system as long as the neutral conductor is not grounded on the load side of the transformer. If a grounding occurs, however, the voltage pulses produce a current in the neutral conductor which does not appear in any of the line conductors. A consequential imbalance is detected by the ground fault sensing means and causes the contacts to open, interrupting the flow of current in the distribution system.
Another known arrangement discloses an electric circuit breaker including highly sensitive ground fault responsive means. The means includes a differential transformer with a toroidal core fabricated of a magnetic material. Phase and neutral conductors pass through an opening in the toroidal core, forming single turn primary windings. The differential transformer also includes a secondary winding comprising a plurality of turns wound on the toroidal core. This secondary winding is connected to a solenoid assembly comprising an armature, an operating coil and a frame mounted on a casing. The armature is adapted for movement between an extended position and a retracted position in response to energization of the operating coil. A latch hook is attached to the armature and disposed for engaging the armature member of the actuator assembly. Thus, energization of the operating coil causes the latch hook to draw the armature away from a latch member to initiate tripping of the circuit breaker. In consequence, the solenoid assembly opens the circuit breaker contacts in response to ground fault current on the order of 3 to 5 mA, and therefore is desirable from the standpoint of protecting human life against electrical shock.
Another known GFCI comprises a differential transformer connected to an AC source which produces a voltage output when an imbalance in current flow between the power lines occurs. The voltage output is coupled to a differential amplifier through a coupling capacitor, rectified, current limited and applied to the gate of an SCR. When the SCR conducts, the winding of a transformer connected across the power line is energized, causing two circuit breaker switches to open. A circuit is also provided for closing the switch when the line becomes unbalanced.
Another protection arrangement uses a ground leakage protector including a ground fault release coil controlled by a ground fault detector. The ground fault release coil is normally energized, and is de-energized when a ground fault appears. Upon detection of a ground fault, a restraining latch is disabled resulting in the opening of the circuit breaker.
Yet another protection arrangement uses a unitary circuit breaker of the molded case type including within its casing means sensitive to ground faults, means sensitive to overcurrents, and means sensitive to short circuit currents. All of the aforementioned means act on a common trip latch of the breaker to cause automatic opening when overcurrent is sensed. Also included is a current imbalance detecting foil which energizes a tripping solenoid to release a normally latched plunger to cause tripping. Similarly, a ground fault protection system is known which employs a dormant oscillator which is triggered into oscillation to initiate disconnection of the protected distribution circuit upon occurrence and detection of a neutral to ground type of fault.
While numerous techniques are available for protecting against ground faults, a key concern in the application of GFCIs in residential and commercial environments is GFCI reliability. As long as the GFCI is operating properly, protection is provided against ground faults, preventing electrical shock. In addressing problems of reliability, it must be considered that most GFCIs are connected to premise electrical wiring at installation and thereafter forgotten, the homeowner or contractor assuming they will operate correctly one, five or ten years after they are installed. Unfortunately, this is not necessarily so. GFCI devices are subject to a number of failure modes. For example, GFCIs are susceptible to bad power supply, open current sensing coil winding, integrated circuit failure, shorted or open SCR device, open breaker coil, failed contacts, etc. Therefore, there exists a need for a GFCI capable of communicating to a user whether or not the device is functioning properly any time after installation.
One solution is to incorporate a test button on the face of the GFCI device that when pressed simulates a ground fault. This simulated ground fault is treated by the internal circuitry as if a real fault occurred. All internal components and circuitry are thereby exercised and tested. If the internal mechanism of the GFCI is working properly, the contacts open and power is removed from the electrical circuit protected. Following a test, the GFCI must be reset to its normal operating condition. This could be done by pushing a reset button on the face of the GFCI device. Users would be instructed to test their GFCIs periodically and replace failed devices. The problem with this scheme is that in reality most users do not test their GFCIs on a regular basis if at all, even when the face of the GFCI is labeled with the words `TEST MONTHLY` on its face. Thus, there is a real need for a GFCI device that incorporates the ability to automatically test itself periodically without any user intervention, in addition to reminding the user to periodically test the GFCI manually.
One factor that lowers GFCI reliability, in addition to a user's failing to test the GFCI, is a power outage and the corresponding surge when power is restored. Therefore it would be beneficial for the GFCI to detect power being restored after a sufficiently long power outage and to force the user to subsequently test the device. Power restoration could cause huge spikes of voltage and current to appear on the power line thus creating a possibility of component failure.
Another potential problem arises because GFCIs typically installed prior to the electricity being applied, especially in new construction. Consequently, there is a real possibility that an installer might inadvertently connect the line side of the AC wiring to the load side of the GFCI. While downstream electrical devices are protected, any receptacles built into the GFCI device itself would not be protected, creating a potential hazard. The GFCI then would remain wired incorrectly unless the device was able to detect a miswiring condition. The ability to detect whether line and load sides were reverse wired would increase the safety level of the device. At the time power is initially applied, the GFCI would alert the user by way of a visual and/or audible alarm, in the event a miswiring condition was detected. The visual and/or audible alarm could not be eliminated until the miswiring condition was removed decreasing the probability of incorrect wiring.